VRAM board for Yamaha CX5MII

I've just read your message about the YIS503II after I had completed the workaround.
But, no problem. This way it will work for both the CX5MII and YIS503II.

The only issue is that the keyboard layout is still the original Japanese, ok? The russian kbd layout is just alien for me, sorry.

I published the patched ROMs in the WIP directory of my site. Have fun with your testing.

I forgot to mention: if you test any cartridge games, remember that the mainBIOS cartridge will have to sit in a lower numbered slot than the game, ok? Because the BIOS must boot before the game.

1) For two separate cartridges the connection is easy. But a 27C256 will only complicate things and require extra logic. You need either a 27C512 or a AM29F010.

With that in mind, each cartridge must be like this:

a) Connect D0~D7 of the 29F010 (or 27C512) to the respective D0~D7 pins of the MSX slot
b) Connect A0~A15 of the 29F010 (or 27C512) to the respective A0~A15 pins of the MSX slot
c) If using the 29F010, connect its A16 to GND.
d) Connect the /CE pin of the Flash/EPROM to the /SLTSL pin of the MSX slot
e) Connect the /OE pin of the Flash/EPROM to the /RD pin of the MSX slot
f) If using the 29F010, connect its /WE pin to the /WR pin of the MSX slot

That's it. Now burn the contents into the 29F010 or 27C512 and have fun.

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2) For a single cartridge some additional circuitry is required. Take a look at this schematic.

This time you will need either a 29F010 Flash or a AT28C010 EPROM.

From the schematic of that simple slot expander, you must remove "U7B", and everything that is connected after to its output pins. CN2, CN3, CN4 and CN5 aren't needed. Then proceed the connections like this:

a) Connect D0~D7 of the 29F010 (or 28C010) to the respective D0~D7 pins of the MSX slot
b) Connect A0~A15 of the 29F010 (or 28C010) to the respective A0~A15 pins of the MSX slot
c) Connect A16~A17 of the 29F010 (or 28C010) respectively to the pins 7 and 9 of the U1. (*1)
d) Connect the /CE pin of the Flash/EPROM to the pin-8 of the U6C
e) Connect the /OE pin of the Flash/EPROM to the /RD pin of the MSX slot
f) If using the 29F010, connect its /WE pin to the /WR pin of the MSX slot

That's it. Now burn the contents into the 29F010 or 28C010 and have fun.

*1: Note that I mentioned A17 in that step. Since this pin is NC on the 29F010/28C010, it will not cause any harm and will allow easy future expansion to a 29F020/28C020 if you want.

1) For two separate cartridges, the requisites are:

- Both files must be saved in the 1st half of empty 27C512 chips. The unused areas will be automatically left as FFh by the burning software.
- There's no order for the cartridges to be inserted.
- The main BIOS cartridge must be in a lower slot than any game cartridge. Otherwise the internal BIOS will boot that cartridge before the external BIOS
- If placed in an slot expander, the external Main BIOS must sit in the slot x-0.

2) As a corollary, for a single cartridge the requisites are:

- Place the main BIOS in the lower half of the 28C010
- Place the sub-ROM in the higher half of the 28C010
- This combo cartridge must be installed in a non-expanded slot
- As it contains the Main BIOS, this cartridge must be connected to a lower numbered slot than any game cartridge. Otherwise the internal BIOS will boot that game cartridge before the external BIOS.

No way. It's an integral part of the system. It would be like trying to make a car run without the gear box.

If you just have one breadboard, you can try the single cartridge approach, can't you?

Just re-read your previous post about schematic. Well, what you described requires 5 logic chips and EEPROM. I think I go for a dual cartridge solution or modify the computer so that there are sockets for both ROMs on board.